Background

The prevalence of allergic rhinitis in Europe has been growing during the last several decades. Climate change is directly affecting the main factors driving the airborne pollen levels released by anemophilous plants. A retrospective long-term assessment of pollen abundance in the air is therefore useful for epidemiological studies, to track the biodiversity and ecosystem evolution, and may also help to estimate the species migration tendencies. The existing studies on pollen trends are usually limited to the individual station or specific regions. In the present reanalysis the symbiosis of in-situ measurements and modern modelling tools are used to reconstruct the year-to-year variations of alder (Alnus), birch (Betula), and olive (Olea) levels over Europe from the 1980 till 2022.

The SILAM model (System for Integrated modeLling of Atmospheric coMposition, http://silam.fmi.fi) was used as the main working tool for computation of the reanalysis. This off-line chemistry transport model contains the pollen specific phenological and emission source modules. The pollen season onset and its following propagation were reproduced based on the model internal formulations only. The corrections to the mean pollen productivity were introduced by assimilation of in-situ pollen observations of European Aeroallergen Network (EAN) in 34 countries. In the extended 4-dimensional variational data assimilation procedure, the total seasonal pollen emission intensity was used as the control variable. The data assimilation technology is embedded in the SILAM model code. The meteorological reanalysis ERA5 was used as an input to drive the SILAM computations.

Assimilation of the pollen concentrations was performed separately for every year and each tree. It resulted in a better representation of pollen observations (including those which were not assimilated) then the unconstrained model run. The reanalysis produced the near surface and three-dimensional hourly pollen concentrations, dry and wet deposition, as well as corrected pollen emission fields. Data are provided on the horizontal grid of 700 x 420 grid cells, Mercator projection, resolution 0.1° x 0.1°, longitude range (25W-45E), and latitude range (30N-72N).

The first long-term pollen reanalysis was produced for three allergenic trees in Europe (1980-2022) with the use of extended 4-dimensional variational data assimilation of the observational data of European Aeroallergen Network by the SILAM model. The procedure substantially improved the model representation of theinter-seasonal variations of pollen absolute levels, thus creating the first systematic assessment of pollen abundance in Europe throughout last four decades.